The impregnation of technical ceramic products in order to achieve a colouring of the product is described in DE 2012304. Coloured ceramic products are achieved according to the teaching of the document by impregnating molded parts with aqueous solutions of colourant heavy metal compounds. The molded parts are prepared from suitable ceramic materials such as clay and kaolin which are formed into the wanted shape and treated with heat in the range of 800 to 1400° C. No pigment is present in the molded parts and the colouring is achieved through impregnation with the aqueous solution and subsequent heat treatment.
In WO 00/15580 the impregnation of ceramics is described which comprises a non-coloured metallic oxide of spinel- or rutile-structure (TiO2, SnO2, ZrO2, or ZrSiO4) that serves as a host lattice to fix bivalent or trivalent metallic ions. The ions are introduced through impregnation of an aqueous solution and allow to colour the ceramic. No indication of conditions used for pre-sintering, impregnation and drying are given. All examples concern the impregnation of clay by Ti/Sb/Cr solutions and yield yellow-ocre colours.